The present invention relates to a pattern inspection method and a pattern inspection apparatus. More particularly, the present invention relates to a pattern inspection method and a pattern inspection apparatus for obtaining multiple pattern images by scanning an object to be inspected on which multiple patterns which should be essentially the same are arranged regularly, such as a semiconductor wafer on which semiconductor chips (dies) are formed, and for making a comparison between neighboring patterns.
On a semiconductor wafer, a plurality of identical chips (dies) are formed in a regular arrangement. In the manufacturing process of a semiconductor device, whether a defect occurs in the formed die is inspected during the process or at the end of the process, and information about occurrence of a defect is immediately fed back to the manufacturing process in order to improve the yield. For this purpose, an inspection of the occurrence of a defect in a pattern is generally carried out by capturing the optical pattern of a die. In order to obtain an image with high resolution, the image of a semiconductor wafer is projected onto a one-dimensional image sensor and the image of the semiconductor wafer is formed by relatively moving the semiconductor wafer and the one-dimensional image sensor for scanning. FIG. 1 is a diagram showing an example of a scanning path for capturing the image of a semiconductor wafer 100 on which a plurality of dies 101 are formed in a regular arrangement. As shown in FIG. 1, on the semiconductor wafer 100, a plurality of the semiconductor chips (dies) 101 are formed in a regular arrangement and scanning is carried out along a scanning path 102 for capturing the image of the entire surface of the semiconductor wafer 100. According to this scanning path, after the lower portion of a die in a row is scanned to the right one after another, the upper portion of the die in the same row is scanned to the left, but a modification such as one in which after the lower portion of a die in a row is scanned to the right one after another, the lower portion of a die in the next row is scanned to the left.
FIG. 2 is a diagram for explaining in detail how scanning is carried out. In this example, the image of the die 101 in a row is captured in four scans 102-1 to 102-4. For example, in the case of a memory, each die 101 has a peripheral circuit part 111 and a cell part 112, as shown schematically. The peripheral circuit part 111 has a random (non-repeating) pattern, but the same cell pattern is repeated at a predetermined pitch at the cell part 112.
Inspection methods for detecting a pattern defect include a method in which a captured pattern of each die is compared with a reference pattern, but a general method currently employed is such one in which the corresponding patterns of neighboring dies are compared and when the two patterns coincide with each other, the pattern is judged to have no defect and when not, it is judged that one die has a defect. As described above, two patterns to be compared with each other should be essentially the same and it is rare for a defect to occur, therefore, such an inspection method is possible.
FIG. 3A to FIG. 3C are diagrams for explaining the defect inspection method when the same pattern is arranged regularly, as described above. The comparison between the patterns of two dies, which is made once, is called a single detection. The single detection cannot judge as to which die is defective when the patterns do not coincide with each other. Therefore, the comparison is made twice as shown in FIG. 3A, that is, the comparison between a die and one of neighboring dies is made and then the comparison between the die and the other neighboring die is made. This is called the double detection. When a die is judged to disagree with its neighboring dies on both sides by double detection, the die is judged to have a defect. For example, when the comparison between a die 101-1 and a die 101-2 is made and the result is that the difference in two pieces of image data of a certain part exceeds a threshold value and a disagreement occurs, and then the comparison between the die 101-2 and a die 101-3 is made and the result is that a disagreement occurs at the same part, the part in the die 101-2 is judged to have a defect. Similarly, the comparison between the die 101-3 and a die 101-4 is made, and thus the comparison between neighboring dies is repeated sequentially.
As shown in FIG. 2, the peripheral circuit part 111 has a random pattern, but the cell part 112 has the same cell pattern repeated at a predetermined pitch. Therefore, the peripheral circuit part 111 is inspected by making the die comparison, which is made between neighboring dies as described above, but the cell part 112 is inspected by making the double detection, in which an image is split at the repetitive period P of the cell pattern and a comparison is made sequentially between neighboring cell patterns. This is called cell comparison. FIG. 3B is a diagram for explaining cell comparison in which the cell pattern is repeated at a pitch of P and the comparison between neighboring cell pattern is sequentially made in such a way that the comparison between a cell pattern 121-1 and a cell pattern 121-2 is made, then the comparison between the cell pattern 121-2 and a cell pattern 121-3 is made, and so on. The cell comparison has the advantage of being unlikely to receive the influence of noise caused by the variations in colors of a wafer and the positional deviations of images because patterns are compared, the distance between which is relatively shorter than that in the case of the die comparison, therefore, it is possible for the cell comparison to have a high detection sensitivity. Because of this, it is desirable to make an inspection of the cell part at which the cell pattern is repeated by the cell comparison and make an inspection of the rest of the part, that is, the peripheral circuit part by the die comparison.
When patterns are compared, it is required that the positions to be compared coincide with each other. The cell comparison is made within the pattern of a die and the cell patterns to be compared are near to each other, therefore, it is possible to easily make the positions of two cell patterns coincide with each other if the repetitive pitch of the cell pattern is known. To be specific, images a pitch of P apart from each other are compared successively in the cell part.
In contrast to this, in the die comparison, a comparison is made between dies. As patterns of each die are exposed by a stepper or the like, the precision in the arrangement depends on the precision in movement of a moving mechanism in a stepper or the like and the distortion of a wafer caused during processing, and an error to a certain extent is inevitable. Therefore, the die comparison is made after the positional deviation is detected and corrected so that the patterns to be compared are made to coincide with each other.
FIG. 3C is a diagram for explaining the conventional positional deviation detecting and correcting process. When the die comparison is made, the image area defined by the width of the scan and a predetermined length in the scanning direction is called the frame, and it is general to make a comparison for each frame. In FIG. 3C, reference numbers 1-F1 and 1-F2 denote the frame of the first die and 2-F1 and 2-F2 denote the corresponding frame of the second die. The die comparison is made on a frame basis and the positional deviation detecting and correcting process is also carried out on a frame basis. To be specific, one of the patterns is moved so that the difference between the two patterns at a certain part in a frame is minimized, and in this case the positional deviation corresponds to the quantity of movement, and the comparison is made with the image moved by the quantity of movement. Either way, conventionally, the positional deviation detecting and correcting process was be carried out on the part to be compared immediately before the comparison.
FIG. 4 is a diagram showing the internal configuration for making the die comparison inspection in a conventional pattern inspection apparatus (inspection machine) of a semiconductor wafer. As will be described later, such a configuration is generally realized as an image processing unit by the use of a computer. The configuration and the operation required for making the die comparison inspection are briefly explained below.
An object to be inspected (wafer) 13 is retained by a chuck 12 capable of moving (in the X and Y directions) and adjusting rotation angles (θ) by means of an XYθ stage 11. Light from a lamp 17 is guided to the wafer 13 to be irradiated wherewith via a beam splitter 16, a tube lens 15 and an optical microscope 14, and the reflected image is projected onto a TDI camera 18 via the optical microscope 14, the tube lens 15, and the beam splitter 16. The TDI camera is used in order to increase the scan speed by improving brightness, but a normal one-dimensional CCD camera may be used instead. In the image capturing section configured as above, the wafer 13 is moved at a constant speed in the scanning direction by means of the XYθstage 11, and an image signal is generated from the TDI camera 18 in a cycle in accordance with the moving speed. The generated image signal is converted into multi-value image data in an A/D converter 19.
The image data is sent to an image delaying memory 21 for temporarily storing image data equivalent to a die, a difference detecting section 23 for calculating the difference in image data between two neighboring dies and an automatic picture alignment section (AP processing section) 22 for correcting the positions of the two pieces of image data from an image capturing section 20.
The image delaying memory 21 temporarily stores image data equivalent to a die and outputs the image data, which the AP processing section 22 stores in synchronization with the capture of the image data of the next die, after delaying by an amount equivalent to a die. The AP processing section 22 detects the positional deviation in the images of the same part (the same frame) of the image data of the die (second die) sent from the image capturing section 20 and the image data of the die (first die) preceding by one sent from the image delaying memory 21, that is, two neighboring dies, on a frame basis.
The difference detecting section 23 carries out the positional alignment of the image data of the first die and the second die based on the positional deviation information obtained by the AP processing section 22, compares the gray levels between corresponding pixels, and generates a gray level difference image.
A defect judging section 24 judges a pixel to be a defect candidate which has a gray level difference exceeding a predetermined threshold in the gray level difference image generated by the difference detecting section 23. At this time, it is not possible to specify the die on which the defect candidate exists between the first die and the second die because of the single detection, therefore, a single defect candidate image, which shows the position of the pixel judged to be a defect candidate by two values, is temporarily stored internally and when the image capture of the third die is started after a certain period of time delay, the similar comparison as described above is repeated between the second die and the third die to obtain the similar two-valued single defect candidate image, which is then compared with the stored single defect candidate image between the first die and the second die (double detection). If there exists a defect candidate within a certain allowable distance, the part corresponding to the pixel is stored in a defect candidate storage section 25 as a defect which exists in the second die.
The pattern inspection apparatus shown in FIG. 4 can be realized by the use of an operation circuit, but generally it is realized as an operation processing unit (computer) controlled by software by the use of a processor and memory. FIG. 5 is a diagram showing the hardware configuration of an operation processing unit. As shown schematically, an operation processing unit 30 includes the image capturing section 20, a memory 31 and a processor 34. The memory 31 includes an image memory 32 for storing image data and a defect information memory 33 for storing defect information, and further includes a working memory or the like for the processor 34 to carry out operation work. It is necessary for the image memory 32 to have a capacity for storing image data equivalent to at least one die and, actually, a memory capacity for several frames is required additionally. The processor reads image data from the image capturing section 20 and the corresponding image data of the die preceding by one in the image memory 32 on a frame basis, and carries out positional alignment and differential operation, and makes a judgment of the difference with respect to the threshold. The image data from the image capturing section 20 is overwritten to the image memory 32 one after another.
The configuration for making the cell comparison is almost the same as that for making the die comparison shown in FIG. 4, except in that the image delaying memory has a small capacity because the memory is required to store only one cell pattern and that the AP processing section 22 is not necessary because positional alignment adjustment is not carried out. When a die comparison section and a cell comparison section are realized by the use of the configuration shown in FIG. 5, the memory can be shared.
FIG. 6A is a time chart of a conventional example, showing the relationship of the process of each part when the die comparison is made by using the pattern inspection apparatus shown in FIG. 4. As shown schematically, this is an example where one scan covers four dies. In this example, the double detection is carried out only for the second and third dies during one scan, therefore, a defect can be detected from the second and third dies but not from the first and fourth dies. Various methods for carrying out the double detection for dies on both ends during a scan have been proposed and an example is a method in which the dies on both ends are compared with dies apart by two dies, respectively, or another example is a method in which the dies on both ends are compared with each other. When the same portion of the dies in the next row are scanned in the reverse direction in the next scan, a method is possible in which the dies on both ends are compared with those on both ends of the second row, but in this case, it is required that the stored image data can be read in the reverse direction.
As shown in FIG. 6A, the image capture is carried out successively from the die at one end to the die at the other contained in the same row during one scan. The positional deviation detection and correction and the die comparison are started after a delay equivalent to one die from the image capture and completed after a delay after the image capture of the last die at the end is completed. In other words, when the image capture of the first die is completed and the image capture of the second die is started, the positional deviation detection and correction and the die comparison are started. When the cell comparison is made separately, the die comparison is not required to be made for the cell part, therefore, when the operation speed of the processor is sufficiently high, there may be cases where the process is suspended while image data of the cell part is captured during the die comparison.
FIG. 6B is a time chart in a conventional example, showing the relationship of the process of each part when the cell comparison is made. The cell comparison is started after a delay equivalent to one cell pattern during image capturing of each die and completed after a delay after the image capture of the last cell pattern at the end is completed. Therefore, the cell comparison is completed when the image capture is completed in each die.
Conventionally, when the die comparison inspection and the cell comparison inspection are made for a wafer, the die comparison inspection is made first by scanning the entire surface of the wafer, then the cell comparison inspection is made by scanning the entire surface of the wafer because it is difficult to simultaneously make the die comparison and the cell comparison because of the lack of operation ability. Therefore, there is a problem that two scans of the entire surface of the wafer are required and the inspection time is lengthened.
Japanese Patent No. 3187827 has disclosed a pattern inspection method and a pattern inspection apparatus, which have a die comparison circuit and a cell comparison circuit, classifies an image into a part for the die comparison and a part for the cell comparison, and compares and inspects each part respectively using the corresponding circuit. FIG. 7 is a time chart of the process of the method and apparatus disclosed in Japanese Patent No. 3187827. As shown schematically, the cell comparison is made while capturing the image of each die, and the positional deviation correction and the die comparison are started after a delay equivalent to a die, and completed after the image capture of the last die at the end of the row is completed and a certain period of time elapses. When a pattern inspection apparatus, which makes the die comparison inspection and the cell comparison inspection at the same time during one scan, is made up of the processor and memory with sufficient processing ability as shown in FIG. 5, the time chart of each process is the same as that shown in FIG. 7.
As explained by referring to FIG. 3C, the positional deviation detection and correction of two images, for which the die comparison is made, are carried out on a frame basis. The width of the frame in the scanning direction corresponds to hundreds of pixels, which is as small as several thousandths of the width of a die. This is applicable to the technique disclosed in Japanese Patent No. 3187827. As the die comparison is made on a frame basis and two images to be compared are required to coincide with each other within the frame, the positional deviation detection and correction described above is basically sufficient.
However, there arises a problem that an image of a die has various factors which vary the image of each die, such as variations in color, and the detection of positional deviation is difficult to make when the number of patterns is small and, in an extreme case, the detection of positional deviation is made even for a part where a defect exists and the correction based on the detected result is made, therefore, the positional deviation detection and correction cannot be carried out precisely. Conventionally, the present die and the immediately preceding die are overlapped and coincidence is examined, but when a sufficient incidence cannot be obtained, the data of positional deviation of the preceding frame is, for example, used for correction. However, the preceding frame is very close and thought to have similar variations in color, therefore, the positional deviation detection and correction cannot be carried sufficiently precisely. If the correction of positional deviation is insufficient, the difference between two images becomes large even for parts with no defect, and, conversely, the difference becomes small where a defect exists and, therefore, the precision of detecting defects is degraded. As described above, the precision of the detection and correction of positional deviation are essential for the die comparison.